The analyses of rainfall characteristics form the core of
the hydrological design of rainwater catchment system. Of importance are such
aspects as rainfall amount, rainfall distribution, length of wet and dry
spells, drought severity among others. The coefficient of variation, unlike the
standard deviation, does not depend on the annual totals but only on the degree
of variation between the monthly totals for each year. Rainfall amount and
rainfall distribution are analyzed by calculating the probability of exceedence
for each value according to a plotting position formula devised by Weibull.

The results indicate that in the hydrological design of
rainwater catchment system, two aspects of rainfall are of paramount
importance. These are rainfall amount and rainfall distribution. The hydrological
design leads to the determination of the major parameters of catchment area and
tank size. This study therefore serves to demonstrate adequately that rainfall
amount influence catchment size while rainfall distribution influences the
storage capacity.